One of the practical limitations on stretching a parison in a blowing mold for bi-axial orientation is that parts of the wall of the parison may be at somewhat different temperatures so that the parison wall stretches more on one side than the other and shifts to one side of the blow mold cavity so as to contact the cool wall of the cavity on one side before the parison can be stretched uniformly.
This invention has a restraining element in axial alignment with the core rod and the reciprocating element has an end face that is brought into contact with the parison, preferably before the blowing of the parison begins. As the parison is blown, the end face of the reciprocating element remains in at least a light contact with the parison and holds it on the center line of the blowing mold, while the end of the reciprocating element is held against the parison by a piston force produced by air pressure on the piston, the piston force being slightly less than the force produced by the pressure of the blowing air within the parison.
The surface of the reciprocating element that contacts with the end of the parison during the blowing operation controls the shape of the center portion of the bottom of the container that is formed in the mold cavity.
In the preferred embodiment of the invention, the parison is maintained at or above orientation temperature while on the core rod by circulating temperature-controlling fluid through the core rod.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.